Nitridable steels for cold flow processes

ABSTRACT

There are disclosed herein novel nitridable steels which can be utilized to form parts by severe cold forming processes and thereafter the so-formed parts can be hardened to a high surface hardness by nitriding. The steels are alloys which contain 0.005 - 0.03 weight percent of carbon and at least one element selected from the group consisting of titanium in a weight percent of 0.2 to 3.0, zirconium in a weight percent of 0.1 to 1.0, hafnium in a weight percent of 0.1 to 1.0, vanadium in a weight percent of 0.2 to 3.0, niobium in a weight percent of 0.2 to 3.0 and tantalum in a weight percent of 0.1 to 1.0, or aluminum in a weight percent of 0 to 2.0 alone or in combination with elements of the aforementioned group. In addition, these steels may also contain nickel in a weight percent of 0 to 15.0, silicon in a weight percent of 0 to 4.0 and manganese in a weight percent of 0 to 1.5. The balance of the alloys is iron.

United States Patent Ronay Primary Examiner-Hyland Bizot Attorney, Agent, or Firmlsidore Match; Hansel L. McGee 57 ABSTRACT There are disclosed herein novel nitridable steels which can be utilized to form parts by severe cold forming processes and thereafter the so-formed parts can be hardened to a high surface hardness by nitriding. The steels are alloys which contain 0.005 0.03 weight percent of carbon and at least one element selected from the group consisting of titanium in a weight percent of 0.2 to 3.0, zirconium in a weight percent of 0.1 to 1.0, hafnium in a weight percent of 0.1 to 1.0, vanadium in a weight percent of 0.2 to 3.0, niobium in a weight percent of 0.2 to 3.0 and tantalum in a weight percent of 0.1 to 1.0, or aluminum in a weight percent of to 2.0 alone or in combination with elements of the aforementioned group. In addition, these steels may also contain nickel in a weight percent of 0 to 15.0, silicon in a weight percent of 0 to 4.0 and manganese in a weight percent of 0 to 1.5. The balance of the alloys is iron.

1 Claim, 4 Drawing Figures [75] inventor: Maria Ronay, Briarcliff Manor,

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Dec. 18, 1972 [21] Appl. No.: 316,212

[52] US. Cl....; 75/123 K; 75/123 M [51] Int. Cl. C221: 37/00; C220 39/00 [58] Field of Search 75/123 K, 123 M; 7/123 K, 7/124 [56] References Cited UNITED STATES PATENTS 2.736.648 2/1956 Eckel 75/123 M 3,110.635 11/1963 Gulya 75/123 K 3,162,751 12/1964 Robbins..... 3,239,332 3/1966 Goss 75/123 K VICKERS HARDNESS Fe0.40Ti-0.02C5.5Ni

1 l l l l I00 "/0 PERCENTAGE REDUCTION IN HEIGHT PATENTEUJUH3 1975 FIG. 1

DEPTH (mals) ALLOY Tl =0.55 Ni=3.5%

Si=0.3% o 020.4% 550 Fe: REMAINDER I l l l l 0.5 1 2 5 4 5 NITRIDING TIME (hours) FIG. 2

VICKERS HARDNESS 1800 NiTRIDED AT 600C I I l 0 1 2 5 PATENTEDJUH3 I975 clam I, Fl G 3 VICKERS --Fe0.53Ti-0.05C5.5Ni0.4Mn-O.3Si

HARDNESS -FeO.40Ti0.02C5 5Ni o "Fe0.52Ti0.02C0.4Mn-0.58i 280 m Fe0.55Ti-0.05C 240 l I l 0 1o 20 40 so so so "/0 PERCENTAGE REDUCTION IN HEIGHT TENSILE STRENGTH 4 100 YIELD STRENGTH STRENGTH pm 40 so 20 so ELONGATION 40 0.75 Ti OF ALLOYING ELEMENT FIG.5

0685" REFERENCE AFTER NITRIDING BEFORE NITRIDING FIG.6

VICKERS HARDNESS ALLOY: Fe0.03C-1.0Ti5Ni-0.5 Si-04Mn lllllllllllll 02468i01214161820222426 DISTANCE FROM SURFACE (mils) NlTRlDABLE STEELS FOR COLD FLOW PROCESSES BACKGROUND OF THE INVENTION This invention relates to steels suitable for use for cold flow processes. More particularly, it relates to improved nitridable steels adantageously suitable for such processes.

Generally speaking, many types of steel parts are formed by cold flow processes such as press forming, blanking, piercing, coining, rotary swaging, cold heading and cold extrusion. Other types of steel parts are formed by cold flow processes which are termed severe forming" processes, examples of the latter being deep drawing, contour roll forming, rotary rolling of type wheels, kneading of type characters, etc. To form these parts by such cold flow processes, particularly those of the more severe type, there are required steels characterized by high plasticity and low strength. Such properties have been imparted to steels by maintaining their carbon content as low as possible. Thus, for the more severe forming processes, the carbon content has been maintained at a maximum of 0.15 weight percent. However, in the producing ofa steel part by a cold flow process, the forming step is only one phase which has to be considered. Thus, the high plasticity and low strength required by the forming process prohibits the use ofa finished steel part made thereby in applications where higher strength and wear resistance are required. Accordingly, it is also necessary to increase the strength and surface hardness of the part after the forming process is completed.

To effect such increase in strength and surface hardness of a part after the forming process has been completed, the following technical requirements have to concurrently be fulfilled.

A. Formability 1. High ductility 2. Little work hardening B. Utilization of a case hardening process which 3. Permits close control of the case hardness and depth in a wide range 4. Does not distort the finished part 5. Is economical with respect to both material cost and process time C. Strength properties 6. The case is wear resistant 7. Both the core and the combination of the core and the case exhibit high static, fatigue and impact strength.

Heretofore, one method which has been employed to increase the hardness ofa steel part produced by a cold flow process has been to apply a carburizing heat treatment to the part. This treatment consists of diffusing carbon at a relatively high temperature (860-920C) into the surface of a low carbon steel part, quenching it in water or oil from the carburizing temperature, and thereafter tempering the quenched part at a relatively low (l50l90C) temperature. This method has produced deleterious effects in that the high temperature of the carburizing and the subsequent rapid cooling causes considerable distortions in the hardened parts, the severity of these distortions essentially being dependent upon the shape of the part. Such distortions have to be corrected such as by straightening and grinding operations which are quite expensive. Consequently, the carburizing heat treatment is not suitable for the surface hardening of parts of complicated shapes or parts produced from sheet metal since distortions would be particularly prevalent in the latter types of parts. However, it is just these parts of complex shapes, or parts produced from sheet metal which are most economically manufactured by severe cold flow processes.

The hardness of a carburized and quenched case of a cold formed steel part depends primarily on the carbon content which results from the carbon diffusion thereinto and only secondarily on the presence of eventual carbide forming alloying element. The reason for this primary dependence is that the size of metal carbide particles is relatively large. Accordingly, in small quantities, they do not impart great hardening. Consequently, the hardness of the carburized layer can only be varied within a limited range. In addition, because it is difficult to control carbon diffusion, neither the hardness nor the depth of the carburized layer can be closely controlled.

Another case hardening process which has been employed is nitriding. By nitriding, there is meant the case hardening process wherein atomic nitrogen made available from ammonia or ammonia-nitrogen gas mixtures is diffused into the surface of a nitridable steel. Nitridable steels contain alloying elements that form finely dispersed stable nitrides at the nitriding temperature and these nitrides provide the high hardness of the nitrided case of a steel part. The nitriding temperature is relatively low," (500600C), and, in the nitriding process, this low temperature treatment is followed by slow cooling. Because of the low temperature of the nitriding process and the slow cooling from this low temperature, nitriding results in a minimum of distortions and in most situations, finished parts of complex shapes can be nitrided without the need for subsequent straightening and grinding.

The conventional nitridable steels are medium carbon alloyed steels that are hardened throughout their cross section by quench and tempering prior to their nitriding to increase the overall strength of the steel. The alloying elements present in the nitridable steel in part contribute to this hardenability and in part form nitrides in the case of the part during the nitriding process.

A conventional nitridable steel typically contains alloying elements in the following weight percents.

C 0.35 0.40 wt pct Mn 0.5 0.8 wt pct Mo =O.2 0.4 wt pct Cr= 1.2 1.6 wt pct Al 0.85 1.50 wt pct Si 0.3 0.5 wt pct Of these alloying elements, the C, Mn and Mo promote the overall hardenability of steel. The elements, Cr and Al, are the active nitride forming elements which effect the hardness of the nitrided layer. In this connection, it is to be realized that aluminum and chromium form nitrides that are small enough to impart great hardness to the nitrided case of the steel part only in a quenched and tempered structure that has a high dislocation density, whereby there are provided many nucleation sites. However, this situation does not obtain in an annealed structure. Therefore, the overall hardening prior to nitriding is also necessary for preparing the structure for nitriding.

Among the alloying elements which promote hardenability in general, viz., C, Mn and Mo, the carbon very greatly hinders the diffusion of nitrogen since it occupies the same interstitial position as nitrogen in the crystal lattice. Concomitantly, it hinders the nitriding process. Because of this hindering effect of carbon, the rate of nitrogen diffusion in known conventional nitridable steels is quite slow and it may take as many as fifty hours to produce an 0.020 inch thick case at a temperature of 560C. In addition to this undesirably slow nitridability, parts cannot be formed from these conventional nitridable steels by severe cold flow processes because their medium carbon content results in low plasticity. Accordingly, up to now, no steel parts which are formed by severe cold flow processes have been nitrided even though such nitriding would be advantageous in reducing of distortions and the concomi tant need for subsequent straightening and grinding.

Accordingly, it is an important object of this invention to provide a novel cold formable-nitridable steels.

It is another object to provide steels in accordance with the preceding object which have improved strength and hardness.

PRIOR ART The conventional nitridable steels are medium car bon alloyed steels that are hardened through the cross section by quench and tempering before nitriding to in crease the overall strength of the alloy. Alloying elements partly serve this hardenability and partly form nitrides in the case during the nitriding process. These conventional steels typically comprise 0.35 to 0.40 weight percent of carbon, 0.5 to 0.8 weight percent of manganese, 0.2 to 0.4 weight percent of molybdenum, 1.2 to 1.6 weight percent of chromium, 0.85 to 1.50 weight percent of aluminum and 0.3 to 0.5 weight percent of silicon. Of these alloying elements, the carbon, manganese and molybdenum promote the overall hardenability while chromium and aluminum are the active nitride forming elements which cause the hardness of the nitrided layer. Aluminum and chromium form nitrides which are sufficiently small and impart great hardness to the nitrided case only in a quenched and tempered structure that has a high dislocation density. However, aluminum and chromium are less effective in an annealed structure. The carbon very greatly hinders the diffusion of nitrogen since it occupies the same interstitial position in the crystal lattice as nitrogen. Consequently, it hinders the nitriding process. Because of this high carbon concentration, the rate of nitrogen diffusion into the aforementioned conventional nitridable steels is so slow that it may take as many as 50 hours to produce an 0.020 inch thick case at 560C. In addition, these conventional nitridable steels cannot be formed by severe cold flow processes because their relatively high carbon content results in low plasticity.

SUMMARY OF THE INVENTION In accordance with the invention, there are provided nitridable steels comprising 0.005 0.03 weight percent of carbon. at least one element selected from the group consisting of titanium in a concentration of 0.2 to 3.0 weight percent, zirconium in a concentration of 0.] to 1.0 weight percent, hafnium in a concentration of 0.1 to 1.0 weight percent, vanadium in a concentration of 0.2 to 3.0 weight percent, niobium in a concen tration of 0.2 to 3.0 weight percent, tantalum in a concentration of 0.1 to 1.0 weight percent, or up to 2.0 weight percent of aluminum alone or in combination with elements of the aforementioned group. In addition, these steels may also contain up to 15 weight percent of nickel, up to 4.0 weight percent of silicon and up to 1.5 weight percent of manganese.

The foregoing and other objects, features and advantages of the invention will be apparent from the follow ing more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. 1 shows a grop of semilogarithmic plots which illustrates the effect of nitriding time on the depth of the nitrided layer at various temperatures of the novel nitridable steels, according to the invention;

FIG. 2 is a graph which illustrates the hardness of the nitrided layer of the nitridable steels according to the invention as a function of the free titanium content thereof;

FIG. 3 is a group of curves which illustrate hardness as a function of reduction in height brought about by cold rolling of nitridable steels according to the invention;

FIG. 4 is a group of graphs which illustrates nickel and silicon plus manganese alloying and also their combined effect on the strength properties of nitridable steels according to the invention;

FIG. 5 is a group of graphs which illustrates profiles of a web made by a nitridable steel according to the invention before and after nitriding; and

FIG. 6 is a graph illustrating the hardness of the nitrided layer as a function of the distance from the surface of nitridable steel according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The complex process of forming parts by severe cold flow and hardening them to a high surface hardness by nitriding is enabled to be performed by the cold formable-nitridable steels provided according to the invention. These steels are characterized by a very low carbon content and an alloying element that forms a sub stitutional solid solution with iron. In addition, this alloying element forms a stable carbide and thereby depletes the iron of carbon in interstitial solid solution and forms a very stable nitride with the nitrogen diffusion into the iron, the size of the nitride particle being formed in annealed or cold worked iron being very small (max 40 A) thereby imparting great hardness to the iron. Further, the alloying element forms intermetallic compounds with nickel or silicon thereby providing a capability for overall precipitation hardening. The novel steels, provided according to the invention, may have additional alloying elements to increase strength. They comprise the following compositions, with the balance iron.

1. Carbon 0.005 0.03 weight percent. The reason for this range of carbon is that, since carbon hinders both the diffusion of nitrogen and plastic deformation and because the intent is to bind even the smallest quantities of the carbon in a stable carbide form, the carbon content of the steel has to be as small as possi' ble to prevent isolation of too much of the carbide forming alloying element, and the formation of large quantities of carbides.

Group [VB Group VB.

Ti =02 3.0 wt. pct. V =02 3.0 wt. pct. Zr 0.l A 1.0 wt. pct Nb 0.2 3.0 wt. pct. Hf 0.] l.0 wt. pct Ta 0.] 1.0 wt. pct.

All of these elements of Groups [VB and VB form stable nitrides of cubic symmetry, being isostructural with NaCl. Because of the cubic and metallic" structures of these nitrides, they have a low energy barrier and thus, a high nucleation frequency in iron thereby resulting in very small precipitates that induce great hardness to the nitrided layer. The stability of the nitrides in these groups of elements is quite great and increases progressively from titanium to hafnium and also from vanadium to tantalum. The solubility of these Group IV.B and Group V.B elements in iron at the nitriding temperatures however decreases progressively from titanium to hafnium and from vanadium to tantalum. Because of solubility considerations, titanium, niobium and vanadium are the most significant alloying elements of those set forth. However, all of the elements of Group [VB and Group V.B set forth hereinabove form intermetallic compounds with iron and with nickel to provide a capability for overall precipitation hardening.

a. Effect of the titanium group (lV.B) and the vanadium group (VB) on nitridability Considering the effect, for example, of titanium on nitridability, first, during the alloying process titanium forms titanium carbide, TiC, with the small amount of carbon present in the iron and thereby depletes it from carbon in solid solution. Consequently, in the nitriding step, the nitrogen diffuses into the alloy almost as rapidly as it would in pure iron and the times required for nitriding are quite short as compared to the times required for nitriding for conventional nitridable steels having a medium carbon content.

In FIG. I, there are shown semilogarithmic plots at 500, 550 and 600C of the effect of nitriding time on the depth of the nitrided layer at various temperatures. In these plots, the abscissa is hours and the ordinates are depths in mils. The alloy which is considered is one that contains 0.03 weight percent of carbon, 0.53 weight percent of titanium, 3.5 weight percent of nickel, 0.3 weight percent of silicon and 0.4 weight percent of manganese. It is to be noted that because the atomic weight of titanium is 48 and that of carbon is 12, 5

the weight percent of titanium should be at least four the hardness to the nitrided layer. In this connection,

it is to be noted that the maximum surface hardness of nitrided iron binary alloys containing different nitride forming alloying elements is approximately inversely related to the nitride particle size which is formed. Nitrided iron-titanium (iron-niobium, iron-vanadium) alloys, hardened by TiN, exhibit greater hardness than nitrided iron-aluminum, or iron-chromium alloys, because the size of the titanium nitride particle is the smallest of all the nitride particles formed in annealed iron. The size of titanium nitride particles is less than 15 Angstroms. However, within irontitanium alloys,

the hardness of the nitrided layer depends upon the quantity of the small TiN particles. It has been found that the hardness of the nitrided layer depends linearly upon the titanium content of the alloy and is 5001 HV (Vickers Hardness) for the titanium percentage range set forth hereinabove, i.e., 0.2 3.0 weight percent (at 0.03 percent C).

In FIG. 2 there is shown a graph which sets forth the hardness of the nitrided layer as a function of the free titanium content of the alloy. Because of this effect. i.e., dependency upon the titanium content, the control of the hardness of the nitrided layer within a wide range is readily attained by concomitantly varying the titanium content of the alloy.

b. Effect of titanium group and vanadium group alloy ing elements on the plastic deformation of low carbon steel.

Small diameter interstitial solute atoms, i.e., carbon and nitrogen, form an atmosphere around dislocations thereby restraining their motion and thus restraining plastic deformation. These carbon and nitrogen atoms are removed from solution by titanium since tita nium has a strong affinity to form the carbide and the nitride. Consequently, with the restraining atmosphere removed from the dislocations, the latter move relatively easily.

Thus, titanium first removes the yield point and produces a non-strain-aging steel; and secondly assures easy glide on slip planes whereby relatively large plastic deformation can take place with little work hardening.

FIG. 3 comprises a series of curves which show the results of cold rolling tests on some examples of the novel steels. In these curves, the abscissa is percent reduction in height brought about by cold rolling and the ordinates are Vickers Hardness. These tests indicate that, for the given compositions, there is little work hardening due to cold forming in a considerable range of plastic deformations.

c. Effect of the titanium and vanadium group metals on the strength of iron The hardness and strength of iron (with very low carbon content), which cannot be hardened and strengthened appreciably by quenching, are increased by tita nium in solid solution in the ferrite. In fact, titanium is a very potent ferrite strengthener.

Precipitation hardening of iron-titanium alloys containing less than 4 percent Ti is also possible provided that nickel or silicon is present in sufficient concentrations. The constituent which causes precipitation hardening is Ni Ti or titanium silicide.

3. Aluminum 0-20 wt pct either alone or in addition to the elements of group IV.B or VB. a) Dislocations produced during cold working provide a high density of nucleation sites for aluminum nitride (AlN). Accordingly, in cold worked iron (as contrasted with annealed iron), the sizes of the AlN precipitates are small enough to impart sufficient hardness to the nitrided layer. b) The elements of group [VB and V.B remove interstitial impurities from dislocations. Through this removal and loying and their combined effect on the strength properties of Fe 0.75 Ti- 0.03 C base alloy. in these curves, it can be seen that the tensile strength of iron 0.03 percent C 0.75 percent Ti is increased from 42,000 psi to 95,000 psi while the yield strength is increased from l6,000 psi to 82,000 psi by the addition of 5 percent nickel, 0.3 percent silicon and 0.4 percent manganese. The structure of the alloys is ferrite and a small amount of TiC prior to nitriding. After nitriding, the structure of the nitrided layer is ferrite, TiN and a small amount of TiC. These alloys have a very fine grain size ASTM 8-10 which is substantially insensitive to overheating.

Effect on the nitrided case The presence of nickel in this situtation does not hinder the diffusion of nitrogen, therefore, does not influence the depth or hardness of the nitrided layer. it prevents, however, brittleness of the nitrided layer even at very great hardness and renders the nitriding process insensitive to previous surface oxidation.

b. Nickel 3-l5 wt pct Alloys with 3-15 weight percent nickel content coupled with 1.5 3.0 weight percent titanium content readily lend themselves to precipitation hardening. The constituent which can be employed to cause such precipitation hardening is Ni Ti (or one of the following, viz., Ni,N b, Ni lr, Ni,llf, Ni v, Ni Ta). In this preparation, the alloy is solution annealed at l000C, water quenched, subsequently cold formed, and then manufactured into the final desired shape. The precipitation hardening of the core and the nitriding of the case take place simultaneously (and in the case in competition for titanium) in the machined part in one heat treatment, typically at 500C for 5 hours. As an example, an iron alloy containing 2 percent Ti and 5 percent Ni after such a heat treatment has a 155,000 psi core strength while the maximum hardness of the nitrided layer is 1200 HV. These precipitation hardened nitridable steels, which can be formed by cold flow processes, are advantageously utilizable for tools, dies, etc.

The structure of the steels is ferrite Ni Ti some TiC in the core, and ferrite Ni Ti TiN some TiC in the nitrided case.

5. Si=-4.0wtpctp Mn=0- 1.5 wtpct Both the silicon and manganese increase the strength of iron by solid solution strengthening. ln alloys which contain 1.5-4 percent silicon coupled with 1.5-3.0 percent titanium, precipitation hardening can be effected, the constituent which causes the precipitation hardening being titanium silicide. An iron alloy containing 1.5 percent titanium and 3.5 percent silicon which is water quenched from 1000C and aged for 5 hours at 500C exhibits a tensile strength of 210,000 psi.

6. The sulphur and phosphor content of the alloys is as for any alloyed steel The Nitriding Process The nitriding of the alloys may take place at any temperature between 480C and 700C. The time necessary for achieving a given layer of thickness concomitantly decreases with increasing temperature (FIG. 1 The hardness of the nitrided layer increases with increasing nitriding temperature between 500-600C. Nitriding at 650C produces a nitrided layer of somewhat less hardness than at 600C.

The new alloys may be nitrided in pure ammonia (NH;,) gas or any mixture of ammonia and nitrogen gases, or ammonia and hydrogen gases, provided that the gas mixture contains at least 10 percent ammonia. It is advantageous to use a relatively low percentage of ammonia concentration in order to minimize the formation of brittle iron nitrides and to increase the layer thickness.

Dimensional Change Due to Nitriding The growth resulting from the nitriding of the novel alloys is 0.000] inch for a 0.010 inch thick layer. Distortion is not present.

FIG. 5 shows the profile of a web taken with a suitable recorder before and after nitriding. It is to be noted that both profiles are parallel. The growth is 0.0002 inch, such growth resulting from the sum of two 0.010 inch thick nitrided layers on the opposite sides of the web. There now follows hereinbelow the examples of nitriding of parts made of the novel steels according to the invention.

EXAMPLE 1 Type made for a high speed line printer is manufactured from an alloy comprising from 0.005 to 0.03 percent C, 0.5 0.75 percent Ti, 3.5 percent Ni, 0.3 percent Si, 0.4 percent Mn, and the balance iron. This steel has the following strength properties: tensile strength 78,600 psi; yield strength 62,000 psi; and elongation, 3l percent. The manufacture of a type slug begins with fine flow blanking, continues with kneading of the characters and is completed by a plurality of grinding There is no measurable distortion of the type slug. The uniform growth of about 1 percent of the thickness of the nitrided layer is accounted for in the grinding operation. A tolerance of 0.0003 mil can be kept on the web. The advantage ensuing from the making of this type slug in accordance with this example as compared to making it from carburizing steels is that the high cost of straightening operations to correct distortions is eliminated.

EXAMPLE 2 A gear is made of an alloy comprising 0.005 0.03 percent C, 0.75 1.0 percent Ti, 5.0 percent Ni, 0.3 percent Si, 0.4 percent Mn, and the remainder Fe. The alloy has the following strength properties: tensile strength 95,000 psi; yield strength 82,000 psi; and elongation 25 percent. The fatigue limit in rotating bending is as follows.

Annealed Nitrided polished 70,000 psi l l0,000 psi notched, K 3 35,000 psi 70,000 psi It is evident that nitriding increases the fatigue limit to a substantial extent.

The gear is cut for rough dimensions and then formed by the form flow process for the final dimension and surface finish. It. is thereafter nitrided at 600C for hours to obtain a 17 mils thick layer. The hardness of the nitrided layer is 900 HV or 64 R if the titanium content is 1.0 percent.

FIG. 6 is a curve which illustrates the hardness of the nitrided layer as a function of the distance from the surface.

The advantage of the novel alloys in this application is that the economical form flow" process can be employed and nitriding preserves the final dimensions on the gear. In addition, fatigue and impact strength of the gear is high.

EXAMPLE 3 A matrix for type forming is made from the alloy which comprises 0.005 0.03 percent C, 1.5 2.0 percent Ti, 6.5 percent Ni, and the remainder Fe. in the manufacture of the matrix, the negative of the character is formed into the matrix by a master tool in the solution annealed (soft) alloy. In addition, the overall dimensions of the matrix are obtained by forming. Thereafter, there is applied to the tool a joint precipitation hardening and nitriding heat treatment at 500C for 5 hours. As a consequence of this heat treatment, the core of the tool has imparted thereto a tensile strength of l70,000 psi and the maximum hardness of the nitrided case is 1200 HV, if the titanium content is close to 2.0 percent.

The advantage of using this novel steel alloy is first, the permitting of the cold forming in the manufacturing technology thereby substantially reducing the cost of manufacture, and secondly the increasing of the tool life by the great wear resistance and anti-galling characteristics of the nitrided layer.

EXAMPLE 4 An automobile body or parts of it are made of a steel comprising 0.005 0.03 percent C, 0.3 0.5 percent Ti, 0.3 percent Si, 0.4 percent Mn, and the remainder substantially Fe.

The automobile body is manufactured by conventional press forming technologies and, before its painting (enameling), is nitrided at 500C for 5 hours. The nitriding produces an 8 mils thick corrosion resistant layer having a hardness of about 600 HV.

The advantages which accrue from the use of this novel steel and its nitriding is the increasing of the hardness (elastic limit) of the body thereby increasing its resistance to denting. The nitrided layer prevents corrosion at any location where the surface enamel is damaged. This type of automobile body will corrode only if the 8 mils thick nitrided layer is removed by grinding. Clearly, since corrosion and dents tend to terminate the useful life of automobiles, these novel steels tend to increase their lifetimes substantially. In addition to parts made of sheet metal, these novel steels could be advantageously employed in many types of gears and parts which are employed in the automobile and other mechanical industries.

To summarize the foregoing, some of the advantages presented by the use of the new-formablc nitridable steels provided according to the invention are as fol- 1. Improved plasticity and lesser work hardening and non-strain aging properties in the metal forming processes compared to the formability of conventional (carburizable) low carbon steels. These improved characteristics result in reduced tool wear, greater flow in one step whereby fewer tools are required, the capability of forming higher strength steels, and better surface finish.

2. In the nitriding process itself, as compared to the nitridability of conventional (non-formable) nitridable steels, there results from the use of these novel steels a reduction of nitriding time. Thus, to produce an 0.020 inch thick case, a period such as five hours of nitriding is required at 600C as compared to the 50 hours required for the nitriding of conventional steels. In addition, there is an advantageous increase of case hardness, i.e., a maximum of l600 HV as compared to a maximum of 1150 HV of known steels.

3. In the substituting of parts produced by cold flow and nitriding for parts produced by cold flow and carburizing, the advantages which flow from the use of the novel steels are better control of case hardness and thickness, no distortions whereby no operations are needed after the nitriding step. small and constant growth (l percent of layer thickness), and higher case hardness, i.e., a maximum of 1600 HV as against a maximum of 900 HV. In addition, the nitrided layer is heat resistant and retains its hardness even up to the nitriding temperature as compared to a carburized layer which is not heat resistant. The parts also are characterized by greater wear resistance both at room temperature and at elevated temperature. Furthermore, they exhibit greater fatigue resistance both at room temperature and at elevated temperatures, greater impact strength both at room temperature and at low temperatures, and resistance to atmospheric corrosion, water and alkaline solutions. Also, parts made with the novel steels offer better machinability prior to nitriding and improved weldability prior to nitriding.

The novel steels provided according to the invention are also eminently well suited for carburizing with any conventional carburizing process because of their low carbon content and titanium and nickel alloying. 1n the course of such carburizing, TiC forms and this formation provides for increased hardness and wear resistance of the surface. The Ti and Ni greatly improve the impact strength of the core and the Ni improves its machinability. Thus, some of the advantages of these new steels employed for carburizing as compared to known conventional carburizing steels are greater ease hardness and wear resistance, greater impact strength, and better machinability.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changed in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A steel alloy consisting essentially of 0.005 0.03 weight percent of carbon, 0.5 0.75 weight percent of titanium. 3.5 weight percent nickel, 0.3 weight percent of silicon, 0.4 weight percent manganese and the balance iron.

t s s s m 

1. A STEEL ALLOY CONSISTING ESSENTIALLY OF 0.005 - 0.03 WEIGHT PERCENT OF CARBON, 0.5 - 0.75 WEIGHT PERCENT OF TITANIUM, 3.5 WEIGHT PERCENT NICKEL, 0.3 WEIGHT PERCENT OF SILICON, 0.4 WEIGHT PERCENT MANGANESE AND THE BALANCE IRON. 